This would still be a very useful addition. In my case, for example, I need to change the PTS to a fixed rate of 25 frames per second and also change the audio timestamps by counting samples. I achieve this with -vf 'setpts=N/(25*TB)' -af 'asetpts=N/SR/TB' which works great, but, of course, it needs re-encoding, which is unnecessary and time consuming, especially as this is used via a server-side script.

A bitstream filter version of setpts/asetpts would be ideal. Is it even technically possible?

Please do not reopen ancient tickets.
There is a user mailing list, the input option -r and you can open a new ticket if you want to report a deficiency of this option.
Don't forget to test current FFmpeg git head and to provide a command line together with the complete, uncut console output.